With appropriate electrical loading, photovoltaic solid state semiconductor devices, commonly known as solar cells, covert sunlight into electrical power by generating both a current and a voltage upon illumination. The current source in a solar cell is the charge carriers that are created by the absorption of photons. These photogenerated carriers are typically separated and collected by the use of PN or PIN junctions in semiconductor materials. The operational voltage of photovoltaic devices is limited by the dark current characteristics of the underlying PN or PIN junction(s). Thus improving the power output performance of any solid state solar cell generally entails simultaneously maximizing absorption and carrier collection while minimizing dark diode current.
Detailed balance calculations are typically used to compute the ideal, limiting performance of semiconductor solar cell devices (see for example, C. H. Henry, Limiting Efficiencies of Ideal Single and Multiple Energy-gap Terrestrial Solar Cells, J. Appl. Phys., vol. 51, pp. 4494-4500, August 1980). Two fundamental assumptions are traditionally made in these theoretical calculations. First, it is assumed that the diode dark current is limited by radiative recombination, and that the radiative recombination rate is set by the energy gap of the semiconductor material used to fabricate the device. Second, all of the photons in the incident spectrum with energy above the energy gap of the device material are assumed to create a charge carrier pair that is successfully separated and collected. In practice, neither of these assumptions is achieved. The dark current, and thus the operating voltage, of single junction homojunction solar cells and subcells are typically limited by non-radiative recombination mechanisms such as space charge recombination and majority carrier injection. Non-radiative recombination processes along with reflection losses also limit the current generating capability of single junction devices. Thus practical single junction solar cells have yet to reach the performance levels predicted by detailed balance calculations.
In recent years, multijunction solar cell structures have broken the Shockley-Queisser limit on solar cell performance derived from detailed balance calculations. Multijunction structures employ several different energy-gap materials, typically in separate PN junctions combined within a monolithic III-V material structure. Compared to state-of-the-art single junction GaAs solar cells, two- and three-junction III-V solar cells have roughly one half the current output, but benefit from a greatly increased voltage, which can be a factor of 2.5 to 3× higher, depending on the number junctions used and the individual properties of each junction subcell.
Even with the record breaking efficiency achieved with III-V multijunction solar cells, there remains keen interest in further improving the power output of these devices for both space and terrestrial applications. Therefore, it is desirable to provide for designs that can effectively suppress dark currents in each of the individual junction subcells employed in multijunction devices. Moreover, it is also desirable to provide design strategies and processes that can maximize the photocurrent generating capability of the limiting subcell within each multijunction structure.